High-level Programming Language (Pascal)

Selection Structure

• makes possible the selection of one of a number of alternative actions, depending on the value of a Boolean expression
• can be implemented in Pascal using a If statement or a Case statement

  If statement

• can be used to implement decisions involving two alternatives
• can also be used to implement decision involving several alternatives
• the syntax of the if-then statement (the selection statement with two alternatives)
  
  　　　if <Boolean expression> then
  　　　　<statement> ;
  
  
  • a sequence of statement is executed or by-passed depending on whether a given Boolean expression is true or false
  • if the Boolean expression is true, the specified statement is executed; otherwise it is bypassed
  • in either case, execution continues with the next statement in the program
  • for example,
    
    　　　if Score <= 50 then
    　　　　writeln ('Failed');
    　　　<statement_A>;
    
    

  • the Boolean expression Score <= 50 is evaluated, and if it is true, the word Failed is displayed. Otherwise, the writeln statement is bypassed. In either case, execution continues with the statement following this if statement (i.e. statement_A)
    
    

• the syntax of the if-then-else statement
  
  　　　if <Boolean expression> then
  　　　　<statement_1>
  　　　else
  　　　　<statement_2>;
  
  
  • if the Boolean expression is true, <statement_1> is executed and <statement_2> is bypassed; otherwise <statement_1> is bypassed and <statement_2> is executed
  • in either case, execution continues with the next statement in the program
  • for example,

    　if Score <= 50 then
    　　writeln ('Failed')
    　else
    　　writeln ('Passed');
    　<statement_A>;

    • the Boolean expression Score <= 50 is evaluated, and if it is true, the word Failed is displayed and the else clause writeln ('Passed') is bypassed. Otherwise, the writeln ('Failed') is bypassed and writeln ('Passed') is executed. In either case, execution continues with the statement following this if statement (i.e. statement_A)
      
      
• the statement that appears in an if statement may be any statement (e.g. compound statement, if statement, case statement, repetition statements)
  
  

  Nested if statement

• is an if statement whose then clause or else clause are also if statements
• for example, the following program segment tests whether the number Y is positive, negative or zero

if Y > 0 then
　writeln ('Positive')
else
　if Y < 0 then
　　writeln ('Negative')
　else
　　writeln ('Zero');


Case statement

When there are many alternatives in the selection process, it may be unwise to use the if statements. This is because the program would be lengthy. The case statement can be used to replace any if statement and the layout is more present looking.

• can be used to implement decisions involving multiple alternatives
• the syntax of the case statement
  
  　case <selector> of
  　　<case_label_list_1> : <statement_1>;
  　　<case_label_list_2> : <statement_2>;
  　　　　　　　:　　　　　　　　　　:
  　　<case_label_list_N> : <statement_N>
  　end;
  
  
• the selector must be an ordinal data type
  • has a finite set of value that can be all listed (e.g. integer, Boolean, char, etc.)
  • real and string are not ordinal data type because they cannot be all listed
  • if the alternatives are chosen according to the values of a real or string variable, nested if statement should be used instead
    
    
• the case label list consists of any number of constants or subranges, separated by commas and followed by a colon; for example

  　case refno of
  　　0　　　 :　zerocount := zerocount + 1 ;
  　　1, 2　　:
  　　3 .. 6　 :　writeln ('The reference number is 3 to 6');
  　　7　　　 :　begin
  　　　　　　　　 writeln;
  　　　　　　　　 writeln ('Processing finished')
  　　　　　　　 end
  　end;

• subrange is written as two constants separated by the subrange delimiter '..'
• same value must not appear more then once in the case label lists
  
  

• when the case statement is executed, the selector is evaluated
  
  
  • if this value is in the <case_label_list_i>, then <statement_i> is executed. After the execution, control will be passed to the statement following the reserved word end which marks the end of the case statement
  • if none of the case label lists contains the value of the selector, then either no statement is executed or, optionally, the statements following the reserved word else are executed, for example


　case operator of
　　'*'　: result := A * B;
　　'/'　: result := A / B;
　　'+' : result := A + B;
　　'-'　: result := A - B;
　　else invalid_operator := true
　end;

• using else part of the case statement is an extension to standard Pascal
  
  

• in standard Pascal failure to match a case label would cause a runtime error, therefore, it may be prudent to guard against exceptional values by preceding the case with an if statement to catch values not covered by the case